By regulating the transmission of genetic information, an organism can select specific information, in order to promote cellular differentiation. A diversity of heterogeneous nuclear RNA (HnRNA) molecules represent the transcription products, portions of which are transported into the cytoplasm as mRNA. We have noted in the sea urchin embryo, that three distinct classes comprise the HnRNA and that three, not necessarily homologous, classes of mRNA exist. In order to determine whether or not certain HnRNA molecules serve functions other than precursor to mRNA, we have undertaken to analyze the distinct HnRNA classes with respect to unique and reiterated nucleotide sequences, methylation, turnover properties and homology to mRNA. Differences in the relative amounts of these HnRNA classes at different embryonic stages suggest functional differences bearing on a role in cellular differentiation. The mRNA classes, consisting of histone mRNA and nonhistone mRNAs containing and lacking poly (A), are also being characterized with respect to properties that may affect their degrees of ribosomal loading and rates of initiation of protein synthesis. The synthesis of these mRNA classes and changes in their population during embryonic development are being studied and related to changes in the synthesis of HnRNA and ribosomal RNA, especial attention being paid to developmental changes in RNA processing as well as transcription.